In many applications the final composition of an alloy needs to be estimated. Phase diagram can be employed but it merely gives the proportion of constituents at equilibrium for a large sample. Microscopy of a sample can be used to determine other information, such as how the constituents form, the grain size, and the types of pearlite. This aim of this report is to utilise microscopy to determine the microstructure, and the weight percent of pearlite and ferrite, for different ferrous alloys.
Refer to Professor N. Zhou, ME230 – Control of properties of materials: Laboratory notes- Winter 2001 – 2A Class, University of Waterloo. Pages 24-26
In order to make a direct comparison between the calculated weight ratios of the phases in steel, and the area ratio, the assumption that the volume is proportional to the weight must be valid. To make the comparison the amount of carbon in the metal must be known. Using lever Law the expected weight percent of pearlite and ferrite in the steel can be calculated. To calculate the area ratio on the screen, lines of arbitrary lengths are drawn, and the approximate length of pearlite (or ferrite) along the span of the line is measured. The ratio between the total pearlite lengths over the span of the line will result in the approximate weight percent.
In the preparation of a good quality specimen many problems can be encountered. A common problem is surface scratches. Surface scratches can be minimized by using proper sanding techniques, such as rotating the sample by 90º when proceeding to different grit of sandpaper. Scratches can be further eliminated if fine diamond or alumina polishing suspension is used. Another problem encountered during production is the rounding of edges. Rounded edge cause focusing at the edges to be difficult. If edge retention is important the specimen should by mounted in a hard plastic matrix, such as bakelite. This is done to create a similar hardness between the two interfaces, so that sanding will not eat away at one level more than the other, thus preserving the edges. By using a chemical etch, certain characteristics within the alloy can be enhanced, either by darkening, or lightening. The results of the etch depend on the type of etch and the timing of the etch. Prolong etching or using the wrong etch will create a sample of inferior quality.
Microscopy of a sample can be used to determine the properties of steels. The microstructure of a sample can relate information on the formation of the constituents, the grain size, the weight percent of pearlite and ferrite, and the composition.
In the production of a specimen, scratches and over etching will reduce the quality of the sample, however using the proper techniques these problems can be minimized.
Refer to Professor N. Zhou, ME230 – Control of properties of materials: Laboratory notes- Winter 2001 – 2A Class, University of Waterloo. Pages 19-20.
Hardenabilty refers to the ease with which martensite forms . The Jominy test is a standard test procedure for the hardenabilty analysis of steels.
In part1 of the laboratory, a Jominy test was carried out on following steels:
(ii) Ultimo 4
Figure 1 shows hardenabilty curves for different steels tested. Comparing the effects of different contents, from Figure 1, it can be seen that at Jominy distance 1, X-10 has highest Rockwell hardness (C) value followed by 1090, Keewatin, Ultimo 4, 1045, and Impacto respectively. Since hardenabilty at the quenched end is determined only by carbon content of the steel, it can be seen that higher carbon content increases hardness of steel. Moreover, comparing 1090 and 1045(steels with similar amounts of Mn, P, S, and Si but different C content) over larger Jominy distances, it can be seen that 1090 has overall higher hardness than 1045. Furthermore, the drop in hardness value for 1045 is much higher than it is for 1090. This provides evidence that there is greater likelihood that pearlite forms in 1045, and bainite in 1090. Since, all other alloying elements except carbon are constant, it can be concluded that carbon also increases hardenability of steel. In other words, it slightly shifts the CCT curve to the right.
The alloying elements increase the hardenabilty of steels. This can be seen by comparing the hardenabilty curves of 1045 and Ultimo 4: steels with the same carbon but different alloying element content. Ultimo 4 has higher alloy content than 1045 steel. From Figure 1, it can be distinguished that Ultimo 4 maintains a rather flat hardenabilty curve, compared to 1045 that has a curve that drops off quickly. Initially, Ultimo 4 has fairly consistent hardness meaning that martensite forms in that region. As Jominy distance increases, hardness value drops and remains constant for a while. At larger Jominy distances, hardenabilty further drops a little. This pattern in the hardenabilty curve confirms that martensite, bainite and pearlite forms along the length of specimen.
Conversely, hardness of 1045 steel drops quickly, meaning that little martensite and bainite has formed. However, after falling, the hardness value remains fairly consistent confirming that soft pearlite forms in the region. Hence, from examining the hardenabilty curves of Ultimo 4 and 1045, it can be deduced that increasing the amount of alloying elements in steels with similar carbon content, the TTT and CCT diagrams are shifted, thus decreasing the required quenching rate to produce martensite, and thereby increasing the hardenabilty of the steel (see Figure 2).
The effects of any specific elements on the hardenabilty of steel cannot be distinguished from data obtained from this laboratory. This is because the specimen of steel alloys that were tested have different alloying contents. Therefore, only general conclusions about the effects of alloying elements can be made. In order to distinguish the effects of specific elements samples should be chosen such that all alloying elements are kept constant with the exception of the one in question.
Also, from the analysis of hardenabilty curves, it can be seen that adding alloying elements increases the hardenabilty of steels by shifting the CCT curve, but alloying elements do not affect the hardness of the steels. Carbon content significantly changes the hardness of steels.
The hardenabilty of plain-carbon steel and the effect of alloying elements on the hardenabilty of steel were deduced by performing the Jominy test and analyzing the hardenabilty curves. It is concluded that the maximum hardness obtainable after quenching steel from austenitic state depends on the carbon content of steel. The alloying elements increase the hardenabilty (i.e. ease with which martensite forms) by shifting the TTT and CCT diagrams to longer times, allowing us to obtain all martensite at slow cooling rates. Furthermore, alloying elements increase the hardenabilty of steel, but not the hardness.